1. Field of the Invention
This invention relates to a solid polymer electrolyte fuel cell and, in particular, to a solid polymer electrolyte fuel cell comprising a cell structure that even an auxiliary machine (a supply device) with low electric power consumption can be used to supply air as an oxidant gas.
2. Description of the Related Art
Fuel cells are highly efficient since they are capable of directly transforming a chemical change into electrical energy, and they are global environment-friendly because they exhaust only a small amount of air pollutant (NOx, SOx etc.) since they are operable without burning a fuel containing nitrogen, sulfur etc. The fuel cells include various types, i.e., a polymer electrolyte fuel cell (PEFC), a phosphoric acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC) etc. Among others, the polymer electrolyte fuel cell (PEFC) is expected to be widely used as a power source for automobiles and home, as a power source for mobile devices and as uninterruptible power source in the future.
FIG. 16 is a schematic view showing a principle of power generation in a fuel cell using methanol as a liquid fuel. This is called a direct methanol type fuel cell (DMFC).
In the DMFC, methanol being mixed with water is supplied into a fuel electrode, being ionized into hydrogen ions by a catalyst while generating carbon dioxide (CO2) gas. The hydrogen ions move to a counter electrode side of the solid polymer electrolyte membrane. Then, on an air electrode (=oxidant electrode), electrons generated at the ionization, oxygen as an oxidant and the hydrogen ions react to create water. These sequential reactions allow generation of electric power, whereby electrical energy can be taken out from the fuel cell.
The liquid fuel and air (=oxidant gas) are each supplied to the respective electrodes through a channel member comprising a channel that allows the respective substances to pass through, and the channel member serves to discharge the water and gas produced in the power generation.
FIG. 17 is a schematic view showing the structure of a conventional fuel cell (DMFC). The fuel cell 101 comprises: a solid polymer electrolyte membrane 110; a fuel electrode 111 disposed on one surface of the solid polymer electrolyte membrane 110; an air electrode (=oxidant electrode) 112 disposed on another surface of the solid polymer electrolyte membrane 110 while forming MEA (=membrane of electrolyte assembly) 113 together with the solid polymer electrolyte membrane 110 and the fuel electrode 111; a metal separator (bipolar plate) 115 formed to provide plural fuel channels 114 on one surface of the MEA 113; a metal separator (bipolar plate) 117 formed to provide plural air (oxidant gas) channels 116 on another surface of the MEA 113; and gaskets 118, 119 as a sealant to seal the periphery of the MEA 113 while being interposed between the metal separators 115 and 117. In general, the plural fuel cells 101 are stacked to increase the output of power.
FIG. 18 is a cross sectional view schematically showing a stack structure in the conventional fuel cell. In the conventional fuel cell, the fuel electrode (i.e., anode, shown as “−” in FIG. 18) and the air electrode (i.e., cathode, shown as “+” in FIG. 18) are disposed alternately, i.e., in series.
The DMFC is expected to be used for compact sized mobile devices, which use a secondary cell at present, because it can take out electrical energy by using methanol as a liquid fuel, and it has been practically used in some areas. On the other hand, the PEFC using hydrogen gas as a fuel has been recently considered to be used for automobiles. In the PEFC, to supply hydrogen gas, for example, a reformer is used to produce hydrogen containing gas from methanol or natural gas. However, the cell system must be so large that it cannot be suited to mobile devices.
In contrast, the DMFC has a possibility that its cell system can be considerably downsized because it is capable of taking out hydrogen ions directly from methanol. However, since the DMFC has a lower output density than the PEFC using hydrogen gas as a fuel, the application of DMFC is limited to devices with low electric power consumption at present. In the DMFC, other liquid fuels than methanol such as dimethylether can be used, and the practical use of each liquid fuel has been studied (e.g., JP-A-2002-175817).
JP-A-2002-175817 discloses a fuel cell (DMFC) that a channel is formed to exhaust carbon dioxide (CO2) produced during the power generation on its fuel electrode side so that an equipment for gas-liquid separation becomes unnecessary, whereby the DMFC system can be simplified and be downsized.
In the conventional fuel cell with the stack structure as shown in FIG. 18, there is a demerit that a feeding channel for fuel/oxidant and a discharging channel for exhaust are complicated in structure since the fuel and oxidant must be separately supplied between neighboring fuel cells in order not to be mixed. In this regard, JP-A-2002-544650 discloses a fuel cell that the cathode side or the anode side of the fuel cell is disposed with a certain distance while facing to each other so as to simplify its distribution structure part for supplying oxidant or fuel.
However, the fuel cell of JP-A-2002-175817 has a problem that the air-feeding channel is not sufficiently secured since it is formed by cutting the surface of a separator (bipolar plate) of mold graphite resin. Therefore, a pressure loss may arise in the channel when air is supplied to the stack structure employed to increase the cell output. Due to this, it is needed to supply the air with a higher pressure than the pressure loss, whereby the power consumption of its supplying pump must be increased. This will cause a problem that the power consumption of its auxiliary machine (=air supply pump) driven by the electric output from the fuel cell must be increased so that the output of the whole fuel cell system is reduced. Thus, for DMFC capable of being downsized, it is a serious problem that the power consumption of the auxiliary machine is increased.
The increase in power consumption of the auxiliary machine is caused by that, in supplying the amount of air required for the power generation of the fuel cell, the pressure loss arises in the fuel cell and therefore the supply pressure has to be increased. If the pressure loss in the channel (particularly the oxidant gas channel) of the fuel cell is reduced, a supply device such as a fan could be chosen as the auxiliary machine for ensuring the sufficient amount of air so as to decrease the power consumption. To achieve this, a cell structure is needed to which the fan can be applied.
As described above (FIG. 18), in the conventional stack structure, the electrodes are disposed in series and the neighboring electrodes are different i.e., the fuel electrode and the air electrode. Thus, substances supplied to the neighboring electrodes are different from each other so that the supplying lines for fuel and air are complicated. This causes an increase in manufacturing cost of the fuel cell.
On the other hand, JP-A-2002-544650 discloses the fuel cell that the cathode side or the anode side of the fuel cell is disposed with a certain distance while facing to each other so as to simplify the distribution structure part for supplying oxidant or fuel. However, because the structure of the channel facing to the cathode and the anode is not disclosed therein, it is not guaranteed whether a pressure loss in the channel (particularly on the cathode side, i.e., on the air electrode side) can be reduced.